1. Field of the Invention
The present invention relates to a coil component used as a major component of a common mode choke coil or a transformer and a method of manufacturing the same.
2. Description of the Related Art
Reductions in the size of electronic apparatus such as personal computers and portable phones have resulted in demand for reductions in the size and thickness (height) of electronic components such as coils and capacitors mounted on internal circuits of electronic apparatus.
However, a wire-wound coil obtained by winding a copper wire around a ferrite core has a problem in that it is difficult to make compact because of structural limitations. Under the circumstance, research and development is active on chip-type coil components which can be provided with a small size and a small height. Known chip-type coil components include multi-layer type coil components provided by forming coil conductor patterns on surfaces of magnetic sheet made of ferrite and stacking the magnetic sheets and thin film type coil components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques.
Common mode choke coils are known as thin film type coil components. FIGS. 7A and 7B are sectional views of common mode choke coils 51 taken along a plane including center axes of coil conductors 59 and 61. FIG. 7A shows a common mode choke coil 51 having coil conductors 59 and 61 which are curved in the form of a convex in a top portion thereof when viewed in the coil section. FIG. 7B shows a common mode choke coil 51 having coil conductors 59 and 61 having a rectangular coil section. As shown in FIGS. 7A and 7B, the common mode choke coils 51 have an insulation layer 57 formed by stacking an insulation film between ferrite substrates (magnetic substrates) 53 and 55 which are provided opposite to each other. The coil conductors 59 and 61, which are provided opposite to each other with the insulation film interposed between them and formed in a spiral configuration, are embedded in the insulation layer 57. The insulation layer 57 and the coil conductors 59 and 61 are formed in the order listed using thin film forming techniques.
An opening 63 is formed on an inner circumferential side of the spiral coil conductors 59 and 61 by removing the insulation layer 57. An opening 65 is formed on an outer circumferential side of the spiral coil conductors 59 and 61 by removing the insulation layer 57. Magnetic layers 67 are formed to fill the openings 63 and 65. Further, a bonding layer 69 is formed on the magnetic layers 67 and the insulation layer 57 to bond a magnetic substrate 55.
When the coil conductors 59 and 61 are energized, a magnetic path M is formed such that it passes through the magnetic substrate 53, the magnetic layer 67 in the opening 63, the bonding layer 69, the magnetic substrate 55, the bonding layer 69 again, and the magnetic layer 67 in the opening 65 in the section including the center axes of the coil conductors 59 and 61. The bonding layer 69 is a film having a thickness on the order of a few μm, although it is non-magnetic. Therefore, substantially no leakage of the magnetic flux occurs in this part, and the magnetic path M may be regarded as a substantially closed path.
In order to improve the common mode filtering property of the common mode choke coil 51, strong magnetic coupling must be achieved between the coil conductors 59 and 61. To increase the strength of the magnetic coupling between the coil conductors 59 and 61, it is necessary to increase the numbers of turns of the coil conductors 59 and 61, to reduce the magnetic path length of the magnetic path M, and to space the layers of the coil conductors 59 and 61 at a small and uniform distance. The numbers of turns of the coil conductors 59 and 61 may be increased in a limited region by reducing the conductor width of the coil conductors 59 and 61 and intervals between adjoining parts of the conductors to reduce the pitches of the conductors. However, a reduction in the conductor width results in an increase in the resistance of the coil conductors 59 and 61. Under the circumstance, the ratio between the height and width (aspect ratio) of the coil sections of the coil conductors 59 and 61 may be increased to maintain the areas of the coil sections substantially constant, so that the resistance will not increase.                Patent Document 1: JP-A-2003-133135        Patent Document 2: JP-A-11-54326        Patent Document 3: Japanese Patent Application No. 2003-307372        Patent Document 4: Japanese Patent No. 2011372        
However, as shown in FIG. 7A, when coil conductors 59 and 61 having an aspect ratio of 0.5 or more are formed using an electro-plating process, the top surfaces of the coil conductors 59 and 61 are curved in the form of convexes, and the bottom surfaces have a planar shape. Therefore, the inter-layer distance between the coil conductors 59 and 61 is shortest at the convex parts of the top surfaces of the coil conductors 59 and gradually increases toward both sides of the convexes. As a result, a capacitance (stray capacitance) between the coil conductors 59 and 61 decreases to reduce the degree of magnetic coupling between the coil conductors 59 and 61, which results in a problem in that the common mode filter property is degraded.
A method for suppressing the reduction in the degree of magnetic coupling attributable to the shape of the top surfaces of the coils is to planarize the top surfaces of the coil conductors 59 and 61 using a chemical mechanical polishing process (CMP process) to make the coil sections rectangular, as shown in FIG. 7B. In this case, however, the manufacturing cost is increased because of the need for the step for planarizing the top surfaces of the coil conductors 59 and 61.
As thus described, when it is attempted to improve the degree of magnetic coupling by increasing the numbers of turns of the coil conductors 59 and 61 or decreasing the magnetic path length for the purpose of improving common mode filtering property, the capacitance generated between the coil conductors 59 and 61 decreases to hinder a sufficient improvement of the degree of magnetic coupling. When the top surfaces of the coil conductors 59 and 61 are planarized to increase the capacitance of coupling between the coil conductors 59 and 61, the number of manufacturing steps increases, and this can result in the problem of an increase in the cost of the common mode choke coil 51 through an increase in the manufacturing cost.